Digitized audio signed mixing apparatus, commonly termed a "mixer", also conventionally acts to condition the incoming input signals by modifying their waveform in order to achieve selected effects. Examples of such effects include the so-called "echo" effect where an incoming signal is used as a basis to generate further signals displaced in time by an appropriate phase shift, and the so called "sherwe" effect where a single incoming signal is used to generate a plurality of similar such signals, each slightly different from the first in order to simulate a plurality of different substantially similar signal sources. Such mixing apparatus is used both at live concerts given by musicians or groups of musicians, and in recording studios, and in each case the incoming or input analogue signals are applied to the mixer unit by approriate input lines from associated transducers, which may be microphones in the case of singers or musical instruments, or special purpose transducers such as the magnetic audio transducers fitted to electric guitars.
Very often there are a considerable number of input signals which, for example in the case of a recording studio, must all be applied to two output lines constituting the left and right stereo outputs which will be recorded onto the master recording from which subsequent reproductions will be made. In some circumstances there are more than two output channels for special purposes, for example, at live concerts the output channels may feed amplifiers from which signals are taken for respective left and right banks of loudspeakers, or in recording studios there may be quadraphonic rather than stereophonic recording. Typically, however, each mixer comprises a considerably greater number of input channels than output channels and the mixer acts to direct signals from each input channel in selected proportions to one or more output channels. As mentioned above, the mixer also acts to process or condition the signals in dependence on specific requirements of the technician operating the mixer, and for this purpose the mixer is provided with a large number of control elements at a control panel whereby the operations to be performed on the input signals, such as filtering, attenuation, amplification, etc can be modified by the technician acting on the control elements such as potentiometers, switches and the like, in order to obtain the desired "mix" of input signs in the output. By skilful use of a mixer the technician can also eliminate unwanted noise components, make tone adjustments and balance the level of each input signal so as to produce an overall output which, in his opinion, best represents the performance in aesthetic terms. In doing this, unwanted components can be attenuated whilst others may be accentuated so that, for example, instruments which naturally produce less volume than others can be brought up to the same or similar volume level for the enjoyment of the listener.
Developments in recording techniques and in musical techniques have resulted in the production of mixer units having a very large number of controls for processing each individual input signal, and a correspondingly large number of input channels. Such mixers thus occupy a very large area and the physical manipulations required to effect processing of all the input signals can be arduous and difficult. Furthermore, it is necessary to make a note of the setting of each individual processing control in order to be able to reproduce the setting of a mixer, for example, when concerts are performed at different venues. Problems of repeatability therefore arise, as well as problems of reliability of physically varied analogue signal processing units such as rheostats, capacitors and the like.
The mixer unit very often forms the heart or central core of a recording studio and may be built into the fixed structure. For concerts at separate venues or sites, however, it is necessary to be able to transport the mixer unit from one place to another and to set it up with repeatable connections, at least initially, which can be modified by the sound technician to take account of variations in the acoustics or other constraints applied by the new concert venue.
Apart from the problems of repeatability and durability, problems of noise also arise with analogue mixing units, especially those which have a large number of input channels since the "noise" present on the output channel is directly related to the number of different input channels and switching operations performed on the individual signals within the mixer. In order to overcome the fidelity problem digital processing techniques have been proposed and, indeed, recording media, such as compact disc and digital audio tape, have been produced in which the analogue musical signal is sampled at a high frequency (typically, 43,000 Hertz) to produce a "digitised" signal, namely a train of digital numbers each representing the value of the analogue signal at the associated sampling instant. Difficulties have been encountered, however, in mixing and processing digital signals due to the large number of individual digital numbers which have to be handled. Moreover, conventional mixing techniques for producing a small number of output signals, for example, two stereo channels, from a large number of input channels, have involved the use of a separate mixing section to which all input channels are connected in parallel. This involves limitations on the available functions and prevents the mixer from being enlarged should this become necessary at some stage subsequent to initial installation.
One such digital signal mixing device is described in UK Pat. No. 2 028 055 which describes a digital signal mixing and operating circuit arranged to mix S-channel input digital signals to obtain T-channel output digital signals. This circuit is provided with a digital memory for storing the matrix elements in digital form and includes a matrix element determining circuit for determining the matrix elements in response to a desired mixing ratio of the S-channel input digital signals to be stored in memory.
Such digital signal mixing apparatus has the disadvantage of requiring not only analogue-to-digital conversion on the input lines for converting the incoming analogue signals into digital form, but also digital-to-analogue converters used to reconvert the digital signals into analogue form in order to be able to perform various of the mixing and signal processing operations. Unfortunately, however, analogue-to-digital conversion, like digital-to-analogue conversion involves the introduction of distortions due to non-linearity in the transfer characteristics of the analogue signal mixing circuit and, furthermore, such analogue signal mixing circuits are often affected by external noises which are unwanted in the output from the mixer.
Another disadvantage of known mixing systems is experienced when it is required to treat a number of input channels as a group. For example, the musical item or programme material may include a choir of singers which, because of the large number of individual voices involved, cannot all be catered for using a single microphone, but which necessitates the use of several microphones. Since, however, the choir is treated as a whole in terms of level, and because all the signals from the individual microphones may require the same special effect (such as echo) it is necessary in conventional mixing units to route the plurality of input channels relating to the choir to intermediate or subgroup channels which can then be used to process the signals and produce an output which is then mixed into the final output signal from the mixer.